MSU Extension Vegetables

Hot water seed treatments for vegetable crop species is a preventative management tool for seed-borne diseases. The goal is to heat and maintain water temperature for a specific amount of time. Seeds are soaked in warm water (100 degrees Fahrenheit) for about 10 minutes before going into the hot water for crop-specific times. After the time has passed, they are placed in cool water to rapidly remove the heat. They can then be planted right away, treated further with a fungicide or dried for storage. Specific recommendations have been developed by numerous organizations for certain species of vegetables (Table 1).

The standard tool used to develop these recommendations were lab grade water bath systems which cost upwards of $1,000 or more. However, there are more affordable tools to raise and maintain water temperature in small batches between 5 quarts and 5 gallons at a time. Our objective in this article is share information on heating tools that cost $100 or less (Table 2).

Methods

Heating time

We recorded the time for each heating tool to increase the base temperature of 5 quarts of water from 58 F to 125 F. The 5 quarts of tap water was measured into the crock pot, water bath and three 12-quart stainless steel stock pots. We used 11 quarts of water in the 5-gallon plastic bucket in order to cover the bucket heater with 8 inches of water. A sous vide tool was set into each of the stock pots and the bucket heater was placed into the bucket. The bucket heater and crock pot were controlled by Inkbird thermostats, which were set with high and low set points of 125 F and 123 F, respectively. The crock pot knob was set to “High.”. Each sous vide tool was set to 125 F. Each unit was simultaneously powered-up and a stopwatch started. As each tool reached the target temperature set point, we recorded the time from the stopwatch. We did not use a lid on any container, until after 1 hour when we placed a lid on the crock pot.

On a separate day and location, we also timed the procedure to bring 4 quarts of water to a boil (212 F) with an electric stove top burner, and then adding it to 4 quarts of 58 F water in a 16-quart cooler until the entire solution was 125 F. We also recorded the time elapsed until it fell below 120 F. The entire trial was performed inside a temperature-controlled building with multiple electrical circuits to manage the wattage.

Results and discussion

In the indoor environment, all systems raised water temperature to the target temperature set point and maintained constant for about 20 minutes (Table 2). The fastest heating systems were the 1200 W and 1000 W sous vide tools and 1000 W bucket heater, followed by the 800 W sous vide tool, 1500 W water bath and stove top water added to a cooler. The 340 W crock pot was slowest.

There were some operational differences among the tools that may likely differentiate their performance in other situations, as discussed below. In general, if performing heating treatments with multiple batches of water for seed treatment simultaneously with multiple electronic heating tools, we recommend that each tool be plugged into its own circuit breaker or GFCI outlet. Also, make sure that any external temperature controller is rated for the wattage of the heater.

Water bath

The electric bench-top water bath heats a solution to a specified temperature by heating and maintaining the temperature of the bottom and sides of a container. Water baths can feature precise temperature controls for cooling and heating water. Some water baths feature circulators.

The 1500 W Lab Line Imperial III water bath unit we tested was slow to heat without a circulator. A lid would have hastened heating. The old model did not have an internal thermometer to set temperature, and the controller knobs only provided relative temperature settings, 1–10. We tracked temperature with a mercury thermometer and overshot our temperature setpoint, thus delaying seed treatment until the water cooled. An external thermostat to control power would have worked better than the knob settings without an internal thermometer, however the Inkbird thermostatic controller we used was limited to 1100 W. It is likely that newer digital water bath systems would work better, at a cost.

Crock pot

Crock pots work similar to water baths by heating the bottom and sides of a container of static water to broad ranges of high temperatures and with programs that vary by make and model (e.g., Warm, Low, Med, High, Auto). These tools do not circulate the water. A digital programmable crock pot can allow the user to program the cooking time at one or more temperature ranges. However, all these settings are too hot for seed treatment, and so an external thermostatic controller is needed to cut the power at a lower temperature. A digital crock pot may not turn on again when the controller reconnects the power, but crock pots with a manual switch tend to remain “on” when the power is cut and restored. Some of the newer Instant Pot-type pressure cookers offer accurate temperature control for sous vide cooking and are speculated to work well for this application.

The 340 W crock pot unit we tested was the slowest method to heat water. After one hour, water temperature was not warm enough without a lid. Once a lid was added, temperature raised more quickly. A higher wattage unit would have also been more effective.

Sous vide tools

Sous vide tools, also called water cookers, are portable heating elements that can be fastened to the sides of many types of thin-walled containers made of metal and plastic or set on their bottom. These tools warm surrounding water by passing electricity through a resistant metal heating element. Sous vide cooking tools all feature an onboard temperature controller and most also feature water circulators. All sous vide tools have parameters for minimum and maximum water depth, and maximum volume for optimal heating and circulation.

These were the fast heaters in our trial and all heated the water to 125 F within 21 minutes, but there were some differences in performance. The 1200 W Gourmia and 1000 W VAVA tools had aggressive and loud circulators and temperature settings had to be reset if powered off. The default temperature was in Fahrenheit (F) with these models. The 800 W Synck tool had a gentle circulator with a directional adjustor and remembered its set temperature when power was cut. The temperature on the Synck model was in Celsius (C) by default.

The VAVA required water depths between 2.75 inches and 4.9 inches. The Gourmia required a minimum depth of 2.75 inches and a maximum depth of 5 inches. The Synck required a minimum depth of 2.5 inches and a maximum depth of 6.125 inches. These water levels must be considered when choosing container and seed displacement of water.

All models beeped once the set temperature was reached and continued to maintain heat and circulation, but the Gourmia model beeped continuously unless the timer was set. All models featured internal timers which could be used to maintain accurate seed treatment times. However, no sous vide tool in this test lowered their temperature once their 20-minute timers went off. Instead, they maintained their temperatures.

We checked the onboard thermometers with the Inkbird thermoprobe (Table 3). All tools were within 2 F of the Inkbird at target temperatures, though the Gourmia and Synck models did not measure ice water accurately. However, only the Synck tool included a calibration mode to manipulate a -0.9–0.9 C correction factor.

Other heaters

Aquarium heaters, bird bath heaters, pond heaters and bucket heaters are similar to sous vide tools in that they are portable heating elements. Most of these do not have onboard temperature controls. Bird bath and pond heaters are meant to keep water just above freezing, and aquarium heaters usually will not bring the temperature above 80 F. Some high-wattage bucket heaters can bring water to a temperature that is high enough to treat seed.

We tested a 1000 W bucket heater. It was a fast tool to heat water but would have benefited from circulation due to the minimum of 8 inches of water required to safely use it. There was a 35 F difference between the top water and bottom water in the bucket. The thermostatic controller probe showed that when the top water was at the target temperature, the bottom water remained around 89 F. When the thermostatic controller probe was moved to the bottom, the top water became too hot for seed treatment. When left unregulated, it brought 11 quarts of water up to 210 F. This tool could damage the thermostat probe cable if it were to touch the heating element.

The stove top/cooler method

Standard gas and electric stove tops and outdoor single-burner fryers using various fuels heat water from the bottom of the container and usually do not measure temperature. To treat seeds with this method requires separate volumes of heated water, and cold water that you mix to achieve the proper temperature, and an insulated container to maintain the temperature over the needed time.

We raised 4 quarts to a boil and then offset 4 quarts of 58 F water to bring the solution to 125 F. Note, this would need to be cooled before treating seeds. The pouring action circulated the water and the insulated lid and sides of the Styrofoam cooler maintained temperature above 120 F for 20 minutes.

External temperature controller

For high-wattage bucket heaters and non-programmable crock pots, it is recommended to have a thermostatic controller. The controller plugs into the wall and gates power to the heating element based on the set point temperature you desire. There are many thermostatic controllers available with a similar price and features. They are often marketed to home brewers and pet owners. Some of these tools feature alarms that sound when the temperature setting is reached, or when the low/high thresholds are crossed. For treating seeds, it is important to use a controller with a waterproof probe that can handle the wattage of the heating tool and the probe must be kept away from heating elements and side walls to accurately maintain the proper water temperature.

The Inkbird model we chose for this trial limited out at 1100 W. The temperature defaulted to Celsius without a Fahrenheit option and remembered its temperature setting when unplugged.

Overall recommendations

The best and safest tools to treat seed with hot water will heat, maintain and measure temperature with accuracy. In this test, the sous vide tools offered the best combination of affordability and features to heat 5 quarts of water, measure temperature and time the treatment accurately. Of all the sous vide tools, the Synck brand had the widest depth parameters, lowest energy draw and was the only one with an adjustable circulator, temperature calibration and settings memory after shutoff. It wasn’t the fastest to heat 5 quarts of water, but these other factors made it the best in this trial.

Before going all in, compare the on-board temperature readings of the sous vide tool with another thermometer. In addition, test out this method with a small batch of old seed first. Count out 200 seeds. Sandwich 100 seeds in a wet paper towel and put it in a plastic bag. Treat the other 100 seeds according to the protocols outlined and then put those in a wet paper towel in a separate bag. After a few days, count how many have sprouted to find your germination percentage. If your germination is worse after treatment, then your tool or technique needs some adjustment before moving on to larger batches.

References

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